Lunar and Planetary Institute






Rockets

About Rockets

A rocket essentially is a container propelled in one direction by exhaust going in the opposite direction. Rockets help spacecraft get into space, stay in space, and maneuver in space. The launch and flight of a rocket are governed by Newton's Laws of Motion.

What are the main parts of a model rocket?

Nose cone — The nose cone is the leading, tapered or pointed section of the rocket. It helps reduce aerodynamic drag.

Body tube — The body tube is the central structure of the rocket. It holds the engine, propellant tanks, and payload.

Engine — The engine burns the propellant and converts it to exhaust to provide the force (thrust) to accelerate the rocket.

Fins — The fins help guide the rocket and provide a stabilizing force.

Payload — The payload is the cargo to be delivered by the rocket (such as Hubble Telescope or International Space Station components). In some cases (like for satellites), it is the system being put directly into space.

Parachute — The parachute is released when the spacecraft returns to Earth. The parachute creates drag and slows the descent so that, upon landing, the rocket is not damaged. The model rocket in this activity does not have a parachute because it will not fly high enough to require one. Model rockets that fly over 100 feet high need parachutes.

How do rockets engines work?
There are two main types of rocket engines:  liquid-propellant engines and solid-propellant engines.

Propellant for liquid rockets is stored in large tanks. The propellant components of the fuel (for example, hydrogen) and the oxidizer (for example oxygen) are stored in separate tanks. The propellant is pumped into a combustion chamber, where it is mixed and ignited. The propellant burns, creating gases under high temperatures and pressures. The expanding gases escape through the nozzle at the lower end of the rocket. The nozzle has a narrow throat through which the exhaust is squeezed. The throat limits the amount of gas that can escape, causing the gas to accelerate as it leaves the engine (up to speeds of 5000 to 10,000 miles per hour). As the exhaust is forced out of the nozzle, it propels the rocket in the opposite direction.

In the case of a solid-propellant rocket engine the propellant is a dry mixture of fuel and oxidizer. It is stored in an insulated case. Under normal conditions the propellant does not burn. Upon launch, however, the propellant is ignited and combustion begins. Like the liquid-propellant engines, combustion of the propellant creates hot, expanding gas that escapes through a nozzle at high pressure, generating thrust.

What kind of fuel do rocket engines use?
Most rockets use liquid or solid chemical propellants. A propellant includes fuel — the part that burns — and an oxidizer (like oxygen) that enables combustion. Jets use jet fuel and burn it using oxygen. In space, there is no oxygen, so rockets have to carry their own.

The space shuttle uses liquid hydrogen as its fuel. It also carries liquid oxygen. These are combined in the space shuttle engines to produce water and heat:

2H2 + O2 ® 2H2O + heat

The heat produced by the combustion of hydrogen and oxygen heats up the water (H2O) and creates an immense volume of steam. The steam is released as exhaust and propels the rocket in the opposite direction.

Solid rocket propellants are a dry mixture of fuel (often hydrogen and carbon) and the oxidizing component (oxygen). Only the exposed surfaces of the dry propellant burns, so scientists work to find ways to have as much surface area exposed as possible so that the propellant burns quickly and produces the most exhaust (thrust).

The combustion of liquid-propellant engines can be controlled more easily than solid propellant, so that a lot or a little thrust can be created, depending on whether you are trying to get the rocket off the launch pad and need lots of thrust, or trying to make a small alteration to the rocket's direction and need only a little thrust. Once a solid rocket propellant is ignited, it burns all at once, making it challenging to limit the amount of combustion. However, lighter engines can be made if solid rocket propellant is used; liquid is heavier and requires more massive rockets and engines. Solid rocket propellant is also easier to store. Liquid propellants such as liquid hydrogen and oxygen require complex cooling and storage systems. Solid rocket propellants are common for military uses such as missiles — and are the most common propellants for model rockets. Liquid-propellant systems are more commonly used by the space industry for human and non-human research and exploration.

What are Newton's Laws of Motion?
In 1687 Isaac Newton developed his Laws of Motion. These laws govern the movement of all objects, including rockets. Understanding the Laws of Motion permits NASA scientists and engineers to accurately guide spacecraft across our solar system.

Newton's First Law:

An object at rest will stay at rest.
An object in motion will stay in motion in a straight line at the same speed as long as no force is applied (more accurately, no unbalanced force).

Newton's Second Law:

An object's acceleration is proportional to the force applied to it.
The force to accelerate an object is proportional to the object's mass.

In equation form, if we call the force "F," the object's mass "m," and the acceleration "a," then Newton's Second Law is F = m × a, the most famous form of this fundamental principle of physics. Rocket thrust is a type of force.

Newton's Third Law:

For every action there is an equal and opposite reaction.

How do Newton's Laws of Motion apply to rocket flight?
When a rocket is lifting off from the launch pad, it is because the thrust exceeds the force that is keeping the rocket in place (weight of the rocket and payload caused by Earth's gravity). The thrust of the rocket engine is greater than the weight of the rocket and the net force accelerates the rocket away from the pad. This reflects Newton's First Law of Motion, which states that an object at rest will stay at rest as long as no unbalanced force is applied.

When a rocket is being launched, there are two forces acting on it. One is the weight of the rocket, the force generated by the gravitational attraction of Earth on the rocket. The second is thrust, the force that moves the rocket. In general, the heavier the rocket, the more thrust needed to get it off the ground. The amount of thrust is determined by the mass of rocket propellant that is combusted, creating exhaust, and the speed at which the exhaust is vented from the rocket. Newton's Second Law of Motion comes into play here, as force (thrust) = mass × acceleration. This formula can also be used to determine the rate at which a rocket accelerates, because acceleration = force/mass.

The movement of the high-speed exhaust in one direction propels the rocket in the opposite direction. This is Newton's Third Law of Motion in action; for every action there is an equal and opposite reaction. Thrust has to be carefully controlled when rockets or payloads — like satellites — are launched into space orbit. Too much thrust or thrust at the wrong time can cause a satellite to be placed in the wrong orbit. Too little thrust can cause the satellite to fall back to Earth. Thrust is carefully controlled throughout the launch and maneuvering.

Once the model rocket is airborne, another force comes into play — drag. Drag is the rocket's resistance to motion caused by the rocket's movement through air. It depends on several factors, including the density of the air, the shape of the rocket, and the roughness of the rocket's surface. The more resistant to motion a rocket is, the more thrust is needed to propel it. The nose cones of rockets are streamlined to help reduce drag. Model rockets experience drag along their entire flight path because they are moving through Earth's atmosphere. Rockets that move through space do not experience drag because there is no atmosphere.

For a rocket to continue to ascend, the thrust must be greater than the weight of the rocket and any drag forces. Once the model rocket has used all of its fuel, it no longer accelerates. However, Earth's gravity continues to act on this rocket to slow it down. If the rocket's speed is slow enough, Earth's gravity eventually will pull it back to Earth. If the rocket's speed exceeds 17,500 miles per hour, it is going fast enough to go into orbit around Earth. The space shuttle orbits Earth. Satellites also orbit Earth. If the rocket's speed exceeds 25,000 miles per hour, the rocket is able to escape Earth completely and goes into an independent orbit around our Sun. Spacecraft that have escaped Earth are exploring other planets and regions of our solar system. Examples of these are the Mars Exploration Rovers and the Cassini-Huygens spacecraft, currently investigating Saturn and its rings.

What factors help determine rocket stability?
Rockets must be stable in flight — they must be able to fly in a smooth, predictable direction. An unstable rocket may tumble or fly in an undesirable (and potentially dangerous!) direction. Fins help stabilize the rocket. They are lightweight extensions attached to the exterior of the model rocket. They streamline the flow of air and provide a large surface area and help to keep the center of pressure behind the center of mass of the rocket. As rockets have become more advanced, scientists have experimented with fins that they can move during flight to alter the direction of the rocket. Fins only help stabilize a rocket when there is air present. In space, the angle at which exhaust is vented is changed to alter the direction of the rocket.

How do Pop-Rockets work?
When vinegar and baking soda are mixed together, carbon dioxide gas is released in a bubbly reaction.

The acetic acid in the vinegar is neutralized by the sodium bicarbonate base in the baking soda. The reaction has two steps. In the first, carbonic acid is produced. Because the carbonic acid is very unstable, it breaks down in a second reaction to form water and carbon dioxide.

HC2 H3O2 (acetic acid) + HCO3– (bicarbonate ion) ® H2CO3 (carbonic acid) + C2H3O2– (acetate ion)

H2CO3 (carbonic acid) ® H2O + CO2

C2H3O2– (acetate ion) combines with the sodium in the baking soda to form NaC2H3O2 (sodium acetate), which is the leftover liquid after the rocket has launched.

The release of carbon dioxide provides the thrust of the model rocket engine. As the vinegar and baking soda mix, they give off carbon dioxide gas. The pressure of the gas eventually builds up until the container cannot hold the gas and the pressure is released by forcing the lid off the container. This expulsion of exhaust creates thrust to send the model rocket off the ground.

Last updated
May 27, 2009

 

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